Several problems arise when analog filter/amplifier circuits are implemented as part of integrated circuits. First, single-ended signals are often recived at the input of the integrated circuit. However, internal to the integrated circuit, differential signals offer improved transmission and noise characteristics and are more suitable for use in amplification and signal conditioning. Second, in semiconductor integrated circuits resistors are typically formed as a diffusion of one conductivity type in a substrate of a second conductivity type. The diffusion-substrate interface forms a PN junction. The substrate is typically biased to keep the PN junction reverse biased under normal conditions. However, the width of the depletion region of the PN junction changes with changes in bias conditions, affecting the crosssection of the resistor and hence the value of the resistance. For some analog circuits such as amplifier/filters, the change in the resistance of a resistor with changes in bias conditions affects the frequency response and distortion of the circuit. Third, integrated circuit area must be minimized to reduce cost. In amplifier/filter circuits with feedback and multiple pole filtering, the area required for the components implementing the transfer function can be significant.